The knowledge of atmospheric visibility is of great importance to the U.S. Government and to the air travel industries. Visibility is measured by a transmissometer which measures a transmissivity, or the related property, the absorption of light by the atmosphere.
Light beams traversing atmospheric paths are subject to absorption and scattering. The intensity of light beams, and consequently the visibility, traversing atmsopheric paths is reduced by absorption and scattering. Absorption is generally created by the molecular and particle components of the atmosphere physically capturing energy from the traversing light and is often the major cause for reducing intensity. However, diffuse and specular scattering of light energy can become quite significant when the atmosphere contains particulate matter. The greater the absorption and scattering, the less is atmospheric visibility.
Transmissometers are in use at airports to measure the visibility. The usual arrangement is the transmission of a beam of light from a source with a known intensity over a path length of a given distance to a receiving photoelectric cell. The output of the photoelectric cell measures the light received and provides a measure of the atmospheric absorption or the transmissivity. Other transmissometers have measured the difference in intensity of a light beam transmitted over two different path lengths and from these measurements the transmissivity or absorption is calculated.
In addition, photometric techniques have been available to make measurements of transmittance, absorptance or reflectance of a sample medium. Usually, such measurements are made relative to a reference medium and have not been generally applied to visibility oriented measurements in the atmosphere, particularly, in situ measurements in clouds.
Although the above methods have capability for in situ operations none have been found to be applied for airborne measurements of the transmissivity of the atmosphere.